make Engine an inner class

src/main/java/com/github/nkzawa/socketio/client/Engine.java

@@ -1,22 +0,0 @@
-package com.github.nkzawa.socketio.client;
-
-import java.net.URI;
-
-class Engine extends com.github.nkzawa.engineio.client.Socket {
-
-    Engine(URI uri, Options opts) {
-        super(uri, opts);
-    }
-
-    @Override
-    public void onopen() {}
-
-    @Override
-    public void onmessage(String s) {}
-
-    @Override
-    public void onclose() {}
-
-    @Override
-    public void onerror(Exception err) {}
-}

src/main/java/com/github/nkzawa/socketio/client/Manager.java

@@ -385,4 +385,24 @@ public class Manager extends Emitter {
 
         public void call(Exception err);
     }
+
+
+    private class Engine extends com.github.nkzawa.engineio.client.Socket {
+
+        Engine(URI uri, Options opts) {
+            super(uri, opts);
+        }
+
+        @Override
+        public void onopen() {}
+
+        @Override
+        public void onmessage(String s) {}
+
+        @Override
+        public void onclose() {}
+
+        @Override
+        public void onerror(Exception err) {}
+    }
 }